\section{Related Work}
\label{sec:related}
The capability of MOS transistors of operating at frequencies as high
as tens of GHz~\cite{itrs_rfams12} makes it possible the development
of fully integrated RF systems comprising integrated antennas and
transceivers. In fact, as the frequency increases, dimensions of
typical RF devices, such as antennas and inductors, decrease. For
example, an antenna operating at 60~GHz can be as small as
680~$\mu$m in terms of axial length. Based on this, several research
groups have experimentally proven the feasibility of inter- and
intra-chip communication by using existing CMOS
processes~\cite{floyd_jssc02,lin_jssc07,tap_07,yu_mtt14}.  In
particular, in the study conducted in~\cite{tap_07}, in which an
experimental setup based on a test chip has been used, it has been
shown that propagation mechanism in lossy silicon is based, mainly, on
the propagation of surface waves. In the same work, the effects of
metal structures and the contribution of antenna orientation has been
investigated.

Wireless Network-on-Chip (WiNoC) paradigm has been recently proposed
as a CMOS compatible solution~\cite{zhao_tc08} for addressing the
scalability problems affecting the on-chip communication system for
future manycore architectures.  Several WiNoC architectures have been
proposed in literature~\cite{ditommaso_hoti11,deb_tc13,dt_14}. Since
transceivers and antennas consume a relevant fraction of the total
silicon area, in~\cite{deb_tc13} the authors introduce several
criteria to establish the optimum number of wireless interfaces under
performance constraints. In the same work, a new architecture
named~\emph{HmWNoC} has been presented. Such architecture exploits the
\emph{small-world} property~\cite{ogras_tvlsi06} in which the network
is divided in subnets and in which wireless and wire-line shortcuts
can be used for inter-subnet communications. More recently, the same
authors study the impact of various modulation schemes in terms of
silicon area and energy efficiency~\cite{dt_14}.

Other interesting WiNoC alternatives can be found in~\cite{take_tc14}
and in~\cite{abadal_ieeecm13}. In particular, the former proposes a
wireless 3D NoC architecture which uses inductive coupling for
inter-layer communication, while the latter introduces antennas based
on graphene. Graphene-based antenna assures working frequency in the
Tera-hertz band while utilizing lower chip area for antennas as
compared to the metallic counterparts.

%% To conclude this panoramic, for design exploration purposes, 
%% several models to estimate area and energy consumption of main WiNoCs 
%% building blocks can  be found in~\cite{Abadal_tnet14}.
